Maths Paper 1, November 2024, question 2.1.2

The first term of an arithmetic series is $6$ and the common difference is $4$. The series has $15$ terms. It is then extended so that it has $60$ terms in total, and the sum of all $60$ terms is $7440$. Write down an equation, using sigma notation, for the sum of only the new terms that were added to the original series.